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Date:   Mon, 18 Feb 2019 23:40:42 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Network Development <netdev@...r.kernel.org>
Cc:     David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jason Wang <jasowang@...hat.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] net: validate untrusted gso packets without csum offload

On Mon, Feb 18, 2019 at 2:12 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Fri, Feb 15, 2019 at 12:15 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > Syzkaller again found a path to a kernel crash through bad gso input.
> > By building an excessively large packet to cause an skb field to wrap.
> >
> > If VIRTIO_NET_HDR_F_NEEDS_CSUM was set this would have been dropped in
> > skb_partial_csum_set.
> >
> > GSO packets that do not set checksum offload are suspicious and rare.
> > Most callers of virtio_net_hdr_to_skb already pass them to
> > skb_probe_transport_header.
> >
> > Move that test forward, change it to detect parse failure and drop
> > packets on failure as those cleary are not one of the legitimate
> > VIRTIO_NET_HDR_GSO types.
> >
> > Fixes: bfd5f4a3d605 ("packet: Add GSO/csum offload support.")
> > Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
> > Reported-by: syzbot <syzkaller@...glegroups.com>
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>
> This causes false positive drops on virtio-net and tun for these
> packets with gso without csum_off. And on pf_packet with proto 0.
>
> This happens because skb->protocol is set in these callers after
> the call to virtio_net_hdr_to_skb. And the flow dissector relies on
> this to start dissection, not the link layer header (if present).
> Moving this logic forward is too much churn for net, especially since
> eth_type_header pulls the header, requiring additional changes to
> adjust csum_start.
>
> virtio_net_hdr_set_proto() aims to fix this by deriving skb->protocol
> from the gso_type. But unfortunately for UDP it unconditionally
> selects ipv4, which will cause drops for UDP over ipv6.

Suggested fix at http://patchwork.ozlabs.org/patch/1044429/

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